1. Field of the Invention
The present invention relates to a photodetector circuit and a semiconductor device including the photodetector circuit.
2. Description of the Related Art
In a variety of fields, semiconductor devices including circuits (hereinafter, also referred to as “photodetector circuits”) which receive light from the outside and output signals corresponding to the amount of incident light are used.
Examples of photodetector circuits are a photodetector circuit including a CMOS circuit (hereinafter, also referred to as a CMOS sensor), and a CMOS sensor includes a photoelectric conversion element (e.g., a photodiode) which enables current corresponding to the amount of incident light to flow and a signal output circuit which holds a potential based on the amount of light entering the photoelectric conversion element and outputs a signal corresponding to the potential.
Note that a CMOS sensor detects the amount of light entering a photoelectric conversion element by performing, in a signal output circuit including a MOS transistor, an operation in which a potential (also referred to as charge) held in the signal output circuit is initialized (also referred to as a reset operation), an operation in which a potential corresponding to the amount of photocurrent flowing through the photoelectric conversion element is generated (also referred to as a potential generation operation), and an operation in which a signal corresponding to the potential is output (also referred to as an output operation).
As an example of semiconductor devices including photodetector circuits, an image display device in which a photodetector circuit is provided in each of a plurality of pixels arranged in a matrix can be given (e.g., see Patent Document 1).
In the image display device, in the case where an object to be detected (e.g., a pen or a finger) exists on a display screen, part of light emitted from the image display device is reflected by the object to be detected and the amount of reflected light is detected by the photodetector circuit, whereby a region on the display screen where the object to be detected exists can be detected.
Further, as an example of semiconductor devices including photodetector circuits, a medical diagnostic imaging device provided with a scintillator and a flat panel detector including a plurality of photodetector circuits can be given (e.g., see Patent Document 2).
In the medical diagnostic imaging device, a human body is irradiated with radiation (e.g., X-rays) emitted from a radiation source, radiation which passes through the human body is converted to light (e.g., visible light) by the scintillator, and imaging data is composed by detecting the light with a photodetector circuit included in the flat panel detector, whereby an image of the inside of the human body can be obtained as electronic data.
However, in a semiconductor device which obtains a variety of data with the use of a photodetector circuit provided therein as described above, a signal (also referred to as a detection signal) output from the photodetector circuit is a composite signal including not only a signal needed for obtaining data (also referred to as an essential signal) but also an unnecessary signal (also referred to as a noise signal) in some cases.
For example, in the above image display device, a signal corresponding to “light which is reflected by the object to be detected to enter the photodetector circuit”, which is output from the photodetector circuit, is an essential signal; on the other hand, a signal corresponding to “light (external light) which enters from the outside of the device, such as sunlight or fluorescent light”, which is output from the photodetector circuit, is a noise signal.
Further, in the above medical diagnostic imaging device, since in light emitted by the scintillator, there occurs a phenomenon (what is called afterglow) in which light emission continues even after radiation emission stops, light received by the flat panel detector might include both “light emitted due to radiation emission” and “light emitted by afterglow”.
In this case, a signal corresponding to “light emitted due to radiation emission” which is output from a photodetector circuit is an essential signal; on the other hand, a signal corresponding to “light emitted by afterglow” which is output from a photodetector circuit is a noise signal.
In order to solve the above-described problem in that a detection signal output from a photodetector circuit includes not only an essential signal but also a noise signal, it is effective to remove only a noise signal selectively from a composite signal. To achieve that, for example, as an image display device, a device including photodetector circuits (CMOS sensors) arranged in a matrix is proposed as in Non-Patent Document 1.
In an image display device in Non-Patent Document 1 (see FIG. 3 in Non-Patent Document 1), in each of photodetector circuits (referred to as photosensors in Non-Patent Document 1) arranged in a matrix, a transistor M1, a transistor M2, and a capacitor CINT function as a signal output circuit and an element D1 functions as a photoelectric conversion element.
In addition, after a reset operation and a potential generation operation are performed in the photodetector circuits in odd-numbered rows in a period during which an object to be detected is irradiated with light by turning on a backlight, a reset operation and a potential generation operation are performed in the photodetector circuits in even-numbered rows in a period during which the object to be detected is not irradiated with light by turning off the backlight.
Note that the time interval of blinking the backlight is short, and it can be considered that the object to be detected hardly moves between when the backlight is on and when the backlight is off.
After that, output operations are performed at the same time in the photodetector circuits in two adjacent rows, and a difference between detection signals thereof is obtained. Then, this operation is performed sequentially, so that output operations are performed in the photodetector circuits in all the rows.
A difference between detection signals thus obtained using photodetector circuits in two adjacent rows is an accurate signal including only an essential signal because a signal (noise signal) corresponding to the amount of light entering the photodetector circuit when the backlight is off is removed from a signal (composite signal) corresponding to the amount of light entering the photodetector circuit when the backlight is on.
In other words, a plurality of detection signals (at least two or more detection signals) are obtained using photodetector circuits, and an accurate detection signal is obtained using the plurality of detection signals.